Angiotensin receptor blockers (ARB) used in the treatment of hypertension, heart-failure and other cardiovascular disease conditions prevents activation and signal transduction by the angiotensin type 1 receptor (AT1) for angiotensin II (Ang II). Binding of Ang II to the AT1 receptor activates signal-transduction that leads to mobilization of intracellular calcium, activation of protein kinases, and activation of gene transcription. Yet the molecular mechanism of modulation of these processes by Ang II or ARBs via binding to the AT1 receptor is not clear. Our long-term goal is to understand how the Ang ll-bound AT1 receptor activates the multiple signal transduction process and how ARBs affect this process. Towards this goal, we have developed Ang II- analogs and receptor mutants that activate the ERK-signaling in absence of the G-protein activation. The current proposal will address two issues: (1) how are the AT1-selective ligands guided to the binding pocket in the receptor? and (2) what are the in vivo consequences of the differences between the G-protein mediated and the G-protein independent AT1 receptor signaling? We will determine whether the extracellular loops (ECL) directly participate in the ligand receptor interaction and the activation/inhibition of the AT1 receptor. We anticipate that ECL-2 interacts with the TM domain in an inactive state (R) of the receptor as well as with the Ang II in the activated state (Specific Aim 1). We have discovered that the AT1 receptor engages the beta-arrestin and its fragments in the Ang II activated cells which are absent under other conditions. We will test the hypothesis that site-specific beta-arrestin proteolysis is a novel signaling mode with functional consequences. We will determine the site of beta-arrestin cleavage and its functional consequences in Ang II activated cells (Specific Aim 2). G-protein-dependent AT1 receptor signaling differs from G-protein- independent signaling due to the fact that it causes nuclear translocation of ERK. We will determine the role of Ang ll/AT1 receptor-induced nuclear translocation of ERK 1/2 in activation of gene expression (Specific Aim 3). These studies are essential for a better understanding of the mechanism of Ang II/AT1 receptor action and the development of improved remedies targeting the AT1 receptor.